Magnetically operated display devices include a display or flip element, typically a disc, mounted for rotation about a central axis. A permanent magnet is positioned on the flip element with its magnetic poles substantially in the plane of the flip element and oriented transversely to the axis of rotation. A U-shaped electromagnet is positioned where its poles can attract or repel the poles of the permanent magnet in the flip element. By reversing the electromagnet pole polarity, the element can be flipped over, rotating on its axis. Rotation is limited to approximately 180 degrees by a mechanical stop, generally, a pole piece of the electromagnet. The opposite faces of the flip element, which is generally planar, present different appearances, one from the other, for example, a color difference. Many such devices are used in a matrix, to present indicia, for example, alphanumeric characters, which are readily distinguishable from their background. Devices of this general character are shown in U.S. Pat. Nos. 3,140,553; 3,283,427; 3,295,238; 3,363,494; 3,365,824; 3,518,664; 3,991,496 and 4,531,318.
A problem arises when it is desired to flip a display element if the forces generated between the poles of the electromagnet and the proximate poles of the permanent magnet are equal and opposite about the pivot axis. Then, although the mechanical stop arrangement prevents motion in one direction, there is no certainty that the indicia device will flip in the other desired direction. This difficulty can be overcome by limiting rotation of the indicia element to an angle less than 180 degrees. In such a construction, the fixed poles of the electromagnet act respectively to repel both poles of the permanent magnet on the element providing an additive force in the direction desired for flipping the device.
Another advantage to limiting rotation of the device to less than 180 degrees is that the device may be flipped over with less applied torque as compared to a starting condition where equal and opposite torques tend to be applied. However, this solution to the problem, which assures that the device will flip when the electromagnetic polarity is reversed, adversely affects the appearance of the display to a viewer because the face of the flip element is not truly perpendicular to the most preferred viewing angle, which is at a right angle to the device or matrix wherein the display device is utilized. Thus, as deviation in the turning angle from 180 degrees increases, that is as the turning angle decreases, the torque requirements for flip-over decrease but viewing quality also decreases. A compromise in construction must be made which has inherently undesirable consequences.